The present invention relates to a pump, and particularly relates to a power steering pump for supplying fluid to a power-steering system of a vehicle.
The use of a pump for supplying power steering fluid to a steering system of a vehicle is well known. Typically, such a pump includes a cam ring, a rotor, and vanes carried by either the cam ring or rotor. The vanes act between the cam ring and rotor and define pumping pockets. On relative rotation of the rotor and cam ring, the pumping pockets expand and contract. A reservoir of fluid communicates with the expanding pumping pockets. Fluid is forced from the contracting pumping pockets to the steering system.
Such a power steering pump is driven from the engine of the vehicle. The pump must provide sufficient fluid flow to enable steering to be accomplished at a relatively low predetermined engine speed, such as during vehicle parking. Conventional power steering pumps are designed to have sufficient output at low engine speeds. Also, the conventional pumps include a mechanism to bypass flow from the steering system at pump speeds above a predetermined speed. These mechanisms avoid the pumping of steering fluid through the steering system at an increasing rate as engine speed increases. A number of such mechanisms exist. Typical pumps having such mechanisms are shown in U.S. Pat. Nos. 3,822,965 and 4,014,630. These patents disclose cheek plate unloading pumps.
It is well known that a cheek plate unloading pump bypasses the pump output flow as pump speed increases. In such a pump the cam ring, rotor, and cheek plate are all disposed within a pumping chamber in a housing. The cheek plate is biased toward the rotor and cam ring. When the cheek plate moves away from the rotor and cam ring against the bias, a flow path is created across the rotor and cam ring causing flow from the contracting pumping pockets of the pump to bypass directly to the expanding pumping pockets of the pump. The amount of flow which is bypassed is in proportion to the amount of movement of the cheek plate.
The cheek plate is biased into engagement with the cam ring and rotor by a spring and by fluid pressure in a cavity adjacent to the cheek plate. The fluid pressure in the cavity is communicated to the cavity from the contracting fluid pockets. If the pressure in the cavity is reduced by venting the cavity to the inlet of the pump, then the cheek plate moves to bypass more fluid and the flow to the system supplied by the pump decreases. On the other hand, if the pressure in the cavity increases, the cheek plate will move in a direction toward the cam ring and rotor and decrease the amount of fluid which is bypassed, and thus increase the flow to the system.
In addition to limiting the flow from the pump when pump speed exceeds a predetermined speed, it is desirable to control the flow from the pump according to the demand by the system supplied by the pump. This has been done by sensing the pressure in a conduit directing flow from the pump. This pressure, the system pressure, is relatively low when steering is not occurring. The system pressure rises during a steering maneuver. This increase in system pressure has been used as a control signal to cause a decrease in the amount of fluid bypassed, thereby increasing the flow from the pump.
U.S. Pat. Nos. 3,822,965 and 4,014,630 each disclose a cheek plate pump having a servo valve for controlling the pressure in the cavity adjacent the cheek plate. When the servo valve opens, the cavity is vented to the pump inlet. As a result, the cheek plate moves to a position in which a greater amount of fluid is bypassed.
The servo valve is controlled by fluid pressures acting on it. In particular, the servo valve is controlled by the difference between pressures on opposite sides of a control orifice, one side of which is at system pressure, and the other side of which is at the pressure just downstream of the contracting pumping pockets, i.e., the pump outlet pressure. Passages in the pump communicate system pressure and pump outlet pressure, respectively, to opposite surface portions of the servo valve. The pump also has passages for communicating the cavity pressure to the servo valve and a passage for directing the cavity pressure from the servo valve to the pump inlet for purposes of venting the cavity pressure.
It has also proven desirable to control output flow from a power steering pump so that the flow increases until a predetermined engine speed is reached and thereafter increasing engine speed produces a decreasing flow. This effect of decreasing the pump output flow is termed in the art as creating a "droop" in the output flow. Droop is deemed advantageous in vehicles where it is desired to decrease the amount of assistance provided by a power steering system as vehicle speed increases.